Aromatic hydrocarbons exemplified by benzene, toluene and xylene are conventionally formed from naphtha and coal tar mainly. Concerning the former, petroleum resource is applied as a raw material but low in yield of aromatic hydrocarbons. Concerning the latter, coal is applied as an inexpensive raw material, but it requires a large amount of organic solvent since a solvent extraction method is employed.
As a method for forming hydrogen gas, a steam reforming method for natural gas and naphtha is currently known. However, the steam reforming method requires a high temperature of about 900° C. Further, a large amount of raw material is burned in order to maintain the temperature for reforming, and additionally steam is used three to four times as large as a theoretical-required amount in order to prevent the catalytic activity reduction, thereby consuming an extremely large amount of energy. Additionally, the steam reforming method is burdened with a problem of generating such products upon reforming and burning as to cause global warming, i.e., a large amount of carbon dioxide.
On the other hand, as a process for producing aromatic hydrocarbons such as benzene and naphthalene and hydrogen from lower hydrocarbons, more particularly from methane, a so-called direct-conversion of methane is generally known. The direct conversion of methane is a method for decomposing methane directly on a catalyst in the absence of oxygen gas, in which rhenium carried on zeolite (HZSM-5) is regarded as an effective catalyst (as disclosed in non-patent documents 1 and 2). However, the direct conversion method is burdened with problems of an extreme decrease in catalytic activity due to deposited carbon and of a low conversion ratio of methane.
As one of methods for solving these problems, a method is proposed in patent document 1, in which a direct conversion of methane is carried out in the presence of carbon monoxide or carbon dioxide of a few percents; however, the carbon monoxide or carbon dioxide remains in the product gas so as to burden the purification and separation of a main product, more specifically hydrogen gas, which is not practical.
In order to solve this problem, the other method is proposed in patent document 2. In this method for producing aromatic hydrocarbons and hydrogen, lower hydrocarbons and a gas containing hydrogen or a hydrogen gas are cyclically and alternately brought into contact with a catalyst, thereby forming aromatic hydrocarbons and a high-purity hydrogen without inducing the catalytic activity decrease.
In the method for producing aromatic hydrocarbons and hydrogen, relating to patent document 2, a supply of lower hydrocarbons (such as methane gas) and a supply of hydrogen gas are alternately changed at the same flow rate and at short intervals, e.g., at every five minutes, as shown in FIG. 3 of the present invention (a schedule of supplying hydrogen and lower hydrocarbons, according to conventional techniques), thereby maintaining the effect of the catalyst (see paragraph 0020 of patent document 2). Specifically, it is required to alternately carry out the change of the supply of methane gas and the supply of hydrogen gas at short intervals. Additionally, the reaction is not made while the hydrogen gas flows, so that a substantial period of time for the reaction for forming aromatic hydrocarbons and hydrogen is half of an apparent period of time for forming them. Further, hydrogen gas is used as same amount as methane gas in this method, which is economically disadvantageous.
When the intervals between the alternations of the supply of methane and hydrogen are extended in the above method, e.g., when a the supply of methane is set to 80 minutes and that of hydrogen is set to 20 minutes (as disclosed in paragraph 0022 of patent document 2), producing rates of aromatic hydrocarbons and hydrogen are significantly decreased and additionally a producing fluctuation per hour is large, as compared with the method of the 5-minute interval. In this case, hydrogen gas is used as same amount as methane gas, more specifically a time ratio between methane and hydrogen is 4:1, much more specifically an amount of hydrogen corresponds to 20% by volume of an amount of methane. This method is recognized to have an effect of lasting the catalytic effect; however, aromatic hydrocarbons and hydrogen can not be formed efficiently.    Non-patent document 1: “JOURNAL OF CATALYSIS” 182, 92-103 (1999)    Non-patent document 2: “JOURNAL OF CATALYSIS” 190, 276-283 (2000)    Patent document 1: Japanese Patent Provisional Publication No. 11-60514    Patent document 2: Japanese Patent Provisional Publication No. 2003-26613